We have characterized a Drosophila protein called Rrp1 (Recombination Repair Protein I), which is a homologue of Escherichia coli exonuclease III based on the similarity of their amino acid sequences. Rrp1 is enzymatically similar to its bacterial homologue both when purified from Drosophila embryos and when expressed in E. coli from its cDNA. The protein copurifies with apurinic endonuclease and dsDNA 3'-exonuclease. The apurinic endonuclease activity has been demonstrated using either partially depurinated plasmid DNA or a synthetic oligonucleotide as substrate. The Rrp1 AP endonuclease specific activity is approximately 100,000 u/mg, less than two-fold different than the exonuclease III specific activity. Cleavage occurs on the 5'-side of the abasic sites and leaves 3'-hydroxyl termini, a known characteristic of class II AP endonucleases. Therefore, Class II AP endonucleases such as Rrp1 carry out an essential step in the repair of a common DNA lesion, abasic sites. A phosphodiesterase activity that removes 3'-blocking groups from DNA termini was also recently demonstrated. This enzymatic function is thought to be essential for repair of oxidative damage in vivo. Rrp1 can act to facilitate DNA repair in vivo, since it protects repair deficient strains of E. coli against DNA damaging agent induced lethality. Experiments will be carried out to test for the biological role of Rrp1 in Drosophila, by studying mutagenesis in fly strains that carry differing amounts of Rrp1 protein. In addition to having DNA repair functions, Rrp1 carries out in vitro the DNA strand transfer step that is important in homologous recombination and catalyzes the renaturation of ssDNA. This is a unique property of Rrp1, since other related proteins lack this property. Rrp1 may facilitate recombinational repair at sites of DNA damage, or may have distinct and independent functions in homologous recombination and DNA repair.